Extreme lag in cycles node editor
I think I figured this one out!

It's the rendering of the default interface FONT that causes the lag. It seems it has to do with the file size of the .ttf used by defualt in Blender (~5MB).

So in User Preferences->System->Interface Font, try switching to a .ttf font with a small file size. I used Lucida Sans at 53kb.

I'm not certain as to why the font's file size matters, but IT WORKS.